There are many image processing applications in which related images need to be compared. One is motion measurement in which the position change of a feature between different temporal samples, such as film frames, is measured. Monitoring and quality assurance systems for audiovisual programming need to compare images at different points along a distribution chain. Published UK patent application GB 2 457 694 describes how audiovisual material can be compared so as to confirm the integrity of such a chain. The comparison of television images can be impeded by processes which ‘re-frame’ the picture; a common example is aspect-ratio (width to height ratio) conversion between 4:3 for standard-definition television and 16:9 for high-definition television. Such conversion may involve re-scaling and/or translation (positional shift) of the picture. A common example is the ‘pan-scan’ process where a narrower sub-region is taken from a ‘wide-screen’ image and this sub-region is ‘panned’ across the wide-screen image to follow significant action. Where material is ‘re-purposed’ for display on portable, small-screen devices a small part of the original image may be selected. These processes may vary over time so as to maintain important action within a frame that is smaller than the original frame. In general these processes combine an affine transform of the image with ‘cropping’ in which part of the original picture is discarded.
It is helpful to be able to compare original and transformed images and measure the applied translation and re-scaling. Aspect ratio conversion and reframing can be detected and quantified by locating corresponding image features and comparing their size and position. And, comparison of image features between different images can be used to confirm identity of image content.
Known techniques for image matching include identifying a limited number of ‘interest points’ in the images and comparing these points; U.S. Pat. No. 6,711,293 is an example. However, known feature point definition methods are too complicated for use in real-time broadcast monitoring applications. The present invention provides a novel, robust and simple feature specification technique that is particularly applicable to the determination of differences in position and scale between corresponding portrayed features.